1. Field of the Invention
This invention relates to a simulated stereo from a monaural source sound reproduction system and more particularly to a method and apparatus for creating a stereo-like sound effect from a monaural signal.
2. Description of the Prior Art
A stereo sound effect is produced by ones sensing the direction of the arrival of different sounds through sensing the small differences between the amplitude and phase of the sounds at the locations of the left and right ears. By the stereo effect in sound reproduction it is meant the effect which allows determination of the directions of arrival of sounds by the above means when two independent sound sources are utilized. By the quadraphonic effect in sound reproduction it is meant the effect which allows the determination of the directions of arrival of sounds by the above means when four independent sound sources are utilized. By monaural information or signal it is meant sound information which results at only one ear or identically at both ears and thereby lacks the phase and amplitude difference information necessary to determine the direction of arrival of sound.
Monaural information or signal is available from many sources. If sound is converted into a corresponding electrical signal by a single microphone, then the resulting signal contains monaural information, since, upon reproduction, insufficient information is present in the reproduced sound for the listener to determine the direction of arrival of the various sounds. In like manner, if the sound is converted into corresponding electrical signals by more than one microphone and the resulting electrical signals are combined directly into a single means of transmission or onto a single means of recording, the resultant signal contains no directional information to allow the listener to determine the direction of arrival of the sounds upon reproduction. Examples of such monaural information are the ordinary telephone sound output, the sound output of ordinary amplitude modulated broadcast radio and the sound output of ordinary broadcast television.
In the past there have been attempts to produce simulated stereo sound effects from monaural signals. Some attempts to create directional effects from monaural information have utilized monaural sound reproduced at one speaker slightly before the same sound is reproduced at another speaker, thereby producing a directional effect together with an echo. Other previous attempts to create directional effects from monaural information have utilized monaural sound reproduced at one speaker with the same sound reproduced at another speaker but with approximately a 180.degree. phase shift at all sound frequencies between the two speakers and with little amplitude difference between the sound generated by the two speakers. This results in a noticable "buzz" or "fuzziness" at various frequencies in the reproduced sound, depending on the speaker separation and the relative location of the listener and the speakers. Many listeners find the echo or fuzziness objectionable.